Electrical energy storage devices may fail in operation, and this can result in an uncontrolled release of stored energy that can create localized areas of very high temperatures. For example, various types of cells have been shown to produce temperatures in the region of 600-900° C. in so-called “thermal runaway” conditions [Andrey W. Golubkov et al, Thermal-runaway experiments on consumer Li-ion batteries with metal-oxide and olivin-type cathodes RSC Adv., 2014, 4, 3633-3642].
Such high temperatures may ignite adjacent combustibles thereby creating a fire hazard. Elevated temperature may also cause some materials to begin to decompose and generate gas. Gases generated during such events can be toxic and/or flammable, further increasing the hazards associated with thermal runaway events.
Lithium ion cells may use organic electrolytes that have high volatility and flammability. Such electrolytes tend to start breaking down at temperatures starting in the region 150° C. to 200° C. and in any event have a significant vapour pressure even before break down starts. Once breakdown commences the gas mixtures produced (typically a mixture of CO2, CH4, C2H4, C2H5F and others) can ignite. The generation of such gases on breakdown of the electrolyte leads to an increase in pressure and the gases are generally vented to atmosphere; however this venting process is hazardous as the dilution of the gases with air can lead to formation of an explosive fuel-air mixture that if ignited can flame back into the cell in question igniting the whole arrangement.
It has been proposed to incorporate flame retardant additives into the electrolyte, or to use inherently non-flammable electrolyte, but this can compromise the efficiency of the lithium ion cell [E. Peter Roth etal, How Electrolytes Influence Battery Safety, The Electrochemical Society Interface, Summer 2012, 45-49].
It should be noted that in addition to flammable gases, breakdown may also release toxic gases.
The issue of thermal runaway becomes compounded in devices comprising a plurality of cells, since adjacent cells may absorb enough energy from the event to rise above their designed operating temperatures and so be triggered to enter into thermal runaway. This can result in a chain reaction in which storage devices enter into a cascading series of thermal runaways, as one cell ignites adjacent cells.
To prevent such cascading thermal runaway events from occurring, storage devices are typically designed to either keep the energy stored sufficiently low, or employ enough insulation between cells to insulate them from thermal events that may occur in an adjacent cell, or a combination thereof. The former severely limits the amount of energy that could potentially be stored in such a device. The latter limits how close cells can be placed and thereby limits the effective energy density.
There are currently a number of different methodologies employed by designers to maximize energy density while guarding against cascading thermal runaway.
One method is to employ a cooling mechanism by which energy released during thermal events is actively removed from the affected area and released at another location, typically outside the storage device. This approach is considered an active protection system because its success relies on the function of another system to be effective. Such a system is not fail safe since it needs intervention by another system Cooling systems also add weight to the total energy storage system thereby reducing the effectiveness of the storage devices for those applications where they are being used to provide motion (e.g. electric vehicles). The space the cooling system displaces within the storage device may also reduce the potential energy density that could be achieved.
A second approach employed to prevent cascading thermal runaway is to incorporate a sufficient amount of insulation between cells or clusters of cells that the rate of thermal heat transfer during a thermal event is sufficiently low enough to allow the heat to be diffused through the entire thermal mass of the cell typically by conduction. This approach is considered a passive method and is generally thought to be more desired from a safety vantage. In this approach the ability of the insulating material to contain the heat, combined with the mass of insulation required dictate the upper limits of the energy density that can be achieved.
A third approach is through the use of phase change materials. These materials undergo an endothermic phase change upon reaching a certain elevated temperature. The endothermic phase change absorbs a portion of the heat being generated and thereby cools the localized region. This approach is also a passive in nature and does not rely on outside mechanical systems to function. Typically, for electrical storage devices these phase change materials rely on hydrocarbon materials such as waxes and fatty acids for example. These systems are effective at cooling, but are themselves combustible and therefore are not beneficial in preventing thermal runaway once ignition within the storage device does occur.
A forth method for preventing cascading thermal runaway is through the incorporation of intumescent materials. These materials expand above a specified temperature producing a char that is designed to be lightweight and provide thermal insulation when needed. These materials can be effective in providing insulating benefits, but the expansion of the material must be accounted for in the design of the storage device.
There is therefore an unfulfilled need for a method to limit cascading thermal runaway in energy storage devices that mitigates the problems of previous proposals.